NIECZA_VERBOSE_EXCEPTIONS to print .args.perl in backtraces

lib/Kernel.cs

@@ -1045,6 +1045,8 @@ public class Frame: P6any {
 
         public static readonly bool TraceCalls =
             Environment.GetEnvironmentVariable("NIECZA_TRACE_CALLS") != null;
+        public static readonly bool VerboseExceptions =
+            Environment.GetEnvironmentVariable("NIECZA_VERBOSE_EXCEPTIONS") != null;
 
         public Frame MakeChild(Frame outer, SubInfo info) {
             if (reusable_child == null) {
@@ -1113,6 +1115,27 @@ public class Frame: P6any {
             }
         }
 
+        public Variable GetArgs() {
+            P6any nw = new P6opaque(Kernel.CaptureMO);
+            nw.SetSlot("positionals", pos ?? new Variable[0]);
+            nw.SetSlot("named", named);
+            return Kernel.NewROScalar(nw);
+        }
+
+        public string DescribeArgs() {
+            string ret = null;
+            try {
+                Variable a = GetArgs();
+                Variable sa = Kernel.RunInferior(a.Fetch().InvokeMethod(
+                    Kernel.GetInferiorRoot(), "perl", new Variable[] { a },
+                    null));
+                ret = sa.Fetch().mo.mro_raw_Str.Get(sa);
+            } catch (Exception ex) {
+                ret = "[cannot display arguments: " + ex + "]";
+            }
+            return ret;
+        }
+
         public string ExecutingFile() {
             BValue l;
             SubInfo i = info;
@@ -3258,10 +3281,12 @@ class LastFrameNode {
             while (from != upto) {
                 sb.Append(Console.Out.NewLine);
                 try {
-                    sb.AppendFormat("  at {0} line {1} ({2} @ {3})",
+                    sb.AppendFormat("  at {0} line {1} ({2} @ {3}) {4}",
                             new object[] {
                             from.ExecutingFile(), from.ExecutingLine(),
-                            from.info.name, from.ip });
+                            from.info.name, from.ip,
+                            Frame.VerboseExceptions ? from.DescribeArgs() : ""
+                        });
                 } catch (Exception ex) {
                     sb.AppendFormat("  (frame display failed: {0})", ex);
                 }

perf/perf.TODO

@@ -8,13 +8,15 @@ Experiment with "if non-overriden, then replace method" hooks
 Experiment with making P6any: Variable
 
 Make LISTSTORE off-loop?
-
 push, shift, etc off-loop?
 
 Avoid calling a function for trivial attribute (or parameter) defaults
 
 Improve isa, somehow.
 
-Subquadratic nibbling.
+Subquadratic nibbling.  Also improve nibbler's use of GetKey and push.
+
+Reduce redundant Cursor creation for, e.g., { say 2 }
+Reduce ContextHelper use in ws, or make it faster.
 
 81537 Mu.new attr: 14093 RxOp.zyg,